1. Field of the Invention
The present invention relates generally to the application of powder particles to filamentary materials. It relates in particular to the uniform application of polymer powder particles to spread continuous fiber tows in a fluidized bed unit.
2. Description of Related Art
Thermoplastic composites offer the potential of more attractive mechanical properties at elevated temperatures than do other materials. The primary concern in achieving this potential has been the difficulty experienced in combining thermoplastics with continuous fiber tows to produce a composite prepreg material.
Many previous attempts to apply polymer matrices to filamentary materials have been attempted including: slurry coating, coating from solvent base matrices, film coating and calendaring. The general disadvantage of most prior art methods for producing composite prepreg materials is the non-uniform distribution of the polymer materials throughout the filamentary materials and the difficulty in removal of some species of carrier materials in the polymer matrix in subsequent processing steps. In particular, the high viscosity of polymer melts and the limited solubility of polymer in volatile solvents have ruled out conventional hot melt and solution prepregging methods. This in turn has led to efforts to develop other combining methods such as emulsion, slurry and dry powder. See, e.g., Babu Varughese and John Muzzy, "Combining LARC-TPI and Powder with Carbon Fiber by Electrostatic Fluidized Bed Coating," 21st International SAMPE Technical Conference, Atlantic City, September 1989. The dry powder prepreg processes presently under development contact the tow with powder and either encase, bind, or sinter the powder to the fibers. See especially, J. L. Throne, R. M. Baucom, and J. M. Marchello, "Recent Developments in Dry Powder Prepregging on Carbon Fiber Tow," FiberTex Conference, Clemson University, October 1989; and K. Friedreich, T. Gogera and S. Fakirove, Composite Science and Technology, Vol. 33, pp. 97-120, 1988. Because of the tendency for movement of powder encased with the tow in an extruded tube, and for binder failure with powder loss during weaving, sintering appears to be the preferred method for attaching the powder. The dry processes spread the tow and contact it with powder suspended in air or nitrogen. Investigators at Georgia Institute of Technology and the University of Akron utilize electrostatic force to collect the particles on the tow.
It is accordingly the primary object of the present invention to provide a method for the uniform application of polymer powder particles to filamentary materials in a continuous manner to form a uniform composite prepreg material.